Fully Endoscopic Excision of Vestibular Schwannomas
By JB Eby, M Ocon, Hrayr K. Shahinian MD, FACS
New applications for intracranial endoscopic surgery continue to evolve. The endoscope provides improved visualization of the skull base, where narrow recesses and angled trajectories impair the direct forward view of the operating microscope. Endoscopic surgery allows for a smaller craniotomy, less dissection and minimal retraction, without compromising the goals of the operation.
Articles describing the use of angled endoscopes to assist microscopic removal of vestibular schwannomas suggest that endoscopes allow for complete visualization of the most lateral aspect of the internal auditory canal, identify exposed air cells, and provide more detailed images of the surrounding neurovascular structures.
In this report we describe three fully endoscopic excisions of 2cm-3.5cm vestibular schwannomas via 1.5cm keyhole retrosigmoid craniotomies. The 00 and 300 endoscopes provided excellent exposure, allowing complete visualization of the most lateral aspect of the internal auditory canal, insuring complete tumor removal. The patients had excellent outcomes and were discharged within 72-hours post-operatively.
From our experience we conclude that the endoscope is ideally suited for a minimally invasive retrosigmoid approach to vestibular schwannomas.
Endoscopic Surgery, Acoustic Neuroma, Vestibular Schwannoma
The estimated incidence of vestibular schwannomas based on cadaveric dissections by Schunecht is said to be 570/100,000.1 Nevertheless, many of these tumors fail to become symptomatic during a patient's lifetime. A consensus statement by the National Institutes of Health in 1991 estimated that between, 2-3,000 vestibular schwannomas are diagnosed each year in the Untied States, representing a symptomatic incidence of only 1/100,000. 1,2
Since the first attempts at resection of vestibular schwannomas by Sandifort, Ballance, Cushing, Olivecrona, and Dandy remarkable progress has taken place in the surgical management of these tumors.1 Advances in computerized topography and magnetic resonance imaging have led to an increased recognition of these tumors at an earlier stage. It is now expected that the facial nerve can be salvaged in almost all cases.3,4 Since the majority of these tumors are vestibular in origin, and when small, prior to the onset of significant hearing deficits, complete surgical removal is possible with preservation of hearing.4,5-8 Several articles over the past 30 years have attempted to characterize a size limit or growth rate to aid physicians in deciding which tumors warrant surgical intervention.1,4,7,8 However, many factors must be considered prior to recommending surgery, including the patient's age, co-morbid illnesses, as well as the size and growth characteristics of the tumor.
Three standard surgical approaches to vestibular schwannomas are currently in practice. These include the translabyrinthine, middle fossa, and retrosigmoid (posterior fossa) approaches. Debate continues as to which approach is best suited for large vs. small tumors, which technique allows for complete tumor removal, and which procedure demonstrates the best results for facial nerve and hearing preservation. Several articles have eloquently outlined the advantages and disadvantages of each approach. 4,6,7,8-10
We have found that the retrosigmoid approach allows for the possibility of hearing preservation, provides excellent visualization of the entire tumor, avoids blind dissection behind the facial nerve, and is well tolerated by patients with minimal discomfort.
Disadvantages of the posterior fossa approach include the inability to completely visualize the lateral extent of the tumor within the internal auditory canal (IAC), as well as incomplete visualization of exposed air cells, which may lead to CSF rhinorrhea.9,10 Using the operating microscope with its direct forward view, it is virtually impossible to "look around the corner" due to the oblique angle of the canal in relation to the trajectory of the dissection.3,4,7
Advances in endoscopic technology have allowed for new applications in intracranial surgery. Several reports have highlighted the advantages of endoscopic visualization during pituitary surgery, vascular decompression surgery, ventricular-peritoneal shunt placement, as well as an alternative approach to skull base tumors of the anterior and posterior fossa. Several reports have highlighted the utility of the endoscope to assist microscopic removal of acoustic neuromas.3,4,7 These articles suggest that the endoscope provides improved recognition of exposed air cells, and may allow for more complete tumor removal by direct visualization of the IAC to remove any residual tumor out of the view of the operating microscope.3,4,7 Our group has been performing endoscope assisted or fully endoscopic surgery of the cerebellopontine angle (CPA) for Trigeminal Neuralgia, Hemifacial Spasm, and other CPA tumors since 1996.11,12 We began using the endoscope to supplement our microsurgical resection of vestibular schwannomas in l998. Using the endoscope in this region we found that the maneuverability and angled lenses of the endoscope provide significant advantages in visualizing and accessing the entire tumor, while avoiding injury to the surrounding neurovascular structures. In this report we present three cases where the endoscope was used as the sole imaging modality to perform fully endoscopic removal of vestibular schwannomas.
Patients Materials/and Methods
History: Patient 1
67-year-old male referred by his internist after presenting 1-month prior with a complaint of right-sided hearing loss. On presentation the patient complained of right-sided hearing loss, he denied vertigo, diplopia, or facial asymmetry. The patient had a history of hypertension for which he was taking Hydrochlorothiazide, but denied any other significant past medical history.
On examination the patient's vital signs were within normal limits. The patient was awake and conversant with no focal neurologic deficits other than right-sided hearing loss. All other cranial nerves were intact. The patient had mildly unsteady gait, but a negative Romberg's sign, his strength was 5/5 in all extremities. The remainder of his exam was unremarkable.
An MRI scan revealed a right-sided 3.5cm mass at the cerebellopontine angle, with an appearance consistent with that of a vestibular schwannoma. An audiogram revealed a 55db severe right-sided sensory neural hearing loss.
History: Patient 2
61-year-old white female with a history of right-sided hearing loss and some subtle balance problems. On presentation the patient complained of right-sided tongue numbness but denied any vertigo or facial weakness. She was on no medications and denied any other significant past medical history.
On examination the patient's vital signs were unremarkable, she had no focal neurologic deficits except for the right-sided hearing loss. All other cranial nerves were intact. The patient had a normal gait, negative Romberg sign and 5/5 strength in all extremities. The remainder of her exam was unremarkable. An MRI scan revealed a 3cm mass at the cerebellopontine angle, with an appearance suggestive of a vestibular schwannoma. An audiogram confirmed a 60-70db severe sensory neural hearing loss on the right.
History: Patient 3
39-year-old white female with a six-month history of right-sided hearing loss. On presentation the patient had no other complaints, denied any balance problems or vertigo. The patient was otherwise healthy and on no medications.
On examination the patient's vital signs were unremarkable, she had no focal neurologic deficits except for a clinically obvious hearing loss on the right. All other cranial nerves were intact. The rest of her neurologic exam was also unremarkable. An MRI scan revealed a right-sided 2.5cm mass at the cerebellopontine angle consistent with a vestibular schwannoma.
The patients were taken to the operating room where under general inhalational anesthesia they underwent a 1.5cm retrosigmoid craniotomy and a fully endoscopic resection of their vestibular schwannoma.
The operation begins with the patient supine in a park bench position. The patient's head is secured in a Mayfield 3-pin head clamp. The head is then flexed and slightly rotated away from the side of the tumor. The table is also rotated away from the side of the lesion to provide direct access to the mastoid process.
A 3cm retroauricular incision is performed; this is followed by dissection of the soft tissues of the scalp carried down to the cranium, using electrocautery and periosteal elevators. Hooks are used to retract the skin and soft tissues. An Anspach drill is used to perform a 1.5cm craniotomy just inferior to the confluence of the sigmoid and transverse sinuses. Bone wax is used to fill any air cells entered during the bone dissection. A curvilinear incision is made in the dura, which is then retracted anteriorly and is held in place with sutures. The CSF is allowed to slowly drain. A combination of mild hyperventilation, mannitol and positioning allows the cerebellum to spontaneously retract posteriorly, opening up a narrow path to the cerebellopontine angle. A 2.7 or 4mm zero degree endoscope (Storz, Culver City, CA) endoscope is then guided atraumatically along this path with minimal dissection and almost no retraction to visualize the tumor. An irrigation sheath attached to the endoscope clears blood and debris from the lens, eliminating the time consuming and unsafe practice of removing and re-inserting the endoscope. A rigid pneumatic holding arm secures the endoscope in position, allowing bimanual surgical dissection.
Upon entering the cerebellopontine angle the surgeon conducts a preliminary survey of the surrounding structures including the trigeminal, facial, and lower cranial nerves, as well as the regional vascular anatomy. (Figure 3) The facial nerve is then stimulated, and its response is measured via a facial nerve monitor (Xomed, Jacksonville, FL), which remains in place for the duration of the case to avoid injury to the facial nerve. Once the surrounding critical structures are identified tumor dissection takes place guided by the zero degree endoscope in much the same manner as the microsurgical procedure. (Figure 4) Using microdissecting instruments as well as the CUSA ultrasonic dissector the interior of the tumor is excised. The dura overlying the internal auditory canal is incised and a diamond bur is used to open the internal auditory canal, following the tumor extent laterally within the canal. In cases of smaller tumors with patients having "serviceable hearing" preoperatively, this portion of the dissection should be performed with extreme caution. Several reports have outlined anatomic guidelines to guard against entry into the membranous and bony labyrinth.4,7 Following entry into the IAC the zero degree endoscope is removed and the thirty-degree endoscope is inserted. Tumor dissection within the IAC is guided by the angled endoscope, allowing complete visualization of the lateral extent of the tumor as it is separated from the facial nerve. Exposed air cells are then filled with bone wax.
Frozen section of these tumors in the operating room revealed spindle cells, confirming that the tumors were Vestibular Schwannomas. Once tumor dissection is complete, the facial nerve is once again stimulated to confirm its function. A final survey of the tumor bed and surrounding structures is then performed to insure adequate hemostasis. The dura is re-approximated; the bone flap replaced and secured with a titanium plate, and the scalp is closed in layers.
The patients awoke from the anesthesia in the operating room confirming intact mental status. Cranial nerve examination revealed mild (Grades I-III/VI) facial paresis. The patients had uneventful recovery and were monitored overnight in the surgical intensive care unit. They were transferred to the floor on post-operative day one, were out of bed ambulating and tolerating a regular diet, discharged within 72 hours post-op.
Follow-up demonstrated complete resolution of the facial weakness (Grade I/VI) by 3 months. Repeat MRI 6-16 month's post-op revealed complete excision of the tumors without evidence of reoccurrence .(Figure 5)
Several recent reports using endoscopes as an adjunct to microsurgical dissection have documented the advantages of using endoscopes to survey the surrounding intracranial structures as well as guiding the intracanalicular extent of tumor dissection.3,4,7 We found endoscopic visualization allowed for smaller craniotomies required less dissection and far less cerebellar retraction.
Over the past six years our group has evolved from the standard microsurgical resection of cerebellopontine angle tumors to an endoscope-assisted resection and currently to a fully endoscopic resection of these tumors. The fully endoscopic approach has been used for the microvascular decompression of the trigeminal nerve in patients with trigeminal neuralgia, the facial nerve in patients with hemifacial spasm and the glossopharyngeal nerve in patients with glossopharyngeal neuralgia. The fully endoscopic approach has also been used in the resection of many cerebellopontine angle tumors including meningiomas, schwannomas of the vestibular and trigeminal nerves, neurofibromas, epidermoids, cholesteatomas and CNS lymphomas.11,12 Other published reports have used this approach to clip saccular aneurysms of the vertebrobasilar system.
We believe that the improved exposure of the entire tumor with minimal retraction reduces the risk of injury to the brainstem and surrounding cranial nerves, and results in a complete tumor removal. This minimally invasive technique allowed for the patients' rapid recovery and resulted in minimal post-operative discomfort.
Advances in fiber optic technology, microinstrumentation and minimally invasive techniques have allowed the field of skull base surgery to evolve from traditional neurosurgical, neuro-otologic and craniofacial techniques, to more functional minimally invasive endoscopic approaches that have resulted in shorter hospitalizations and overall excellent outcomes. No doubt future advances will continue to erode into the aura of inaccessibility of the skull base and the mystique of "no-man's land".
- Rosenberg SI. Natural history of acoustic neuromas. Laryngoscope 2000; 110:497-508.
- Acoustic Neuroma National Institutes of Health Consensus Development Conference on Acoustic Neuroma: National Institues of Health, 1991:1-24.
- Kartush JM, Lundy LB. Facial nerve outcome in acoustic neuroma surgery. Otolaryngol Clin North Am 1992;25:623-47.
- Low WK. Enhancing hearing preservation in endoscopic-assisted excision of acoustic neuroma via the retrosigmoid approach. J Laryngol Otol 1999;113:973-7.
- Bremond G, Garcin M, Magnan J. Preservation of hearing in the removal of acoustic neuroma. ('minima' posterior approach by retrosigmoidal route). J Laryngol Otol 1980;94:1199-204.
- Cohen NL, Ransohoff J. Hearing preservation--posterior fossa approach. Otolaryngol Head Neck Surg 1984;92:176-83.
- Magnan J, Barbieri M, Mora R, Murphy S, Meller R, Bruzzo M, Chays A. Retrosigmoid approach for small and medium-sized acoustic neuromas. Otol Neurotol 2002;23:141-5.
- Staecker H, Nadol JB, Jr., Ojeman R, Ronner S, McKenna MJ. Hearing preservation in acoustic neuroma surgery: middle fossa versus retrosigmoid approach. Am J Otol 2000;21:399-404.
- Chen JM, Fisch U. The transotic approach in acoustic neuroma surgery. J Otolaryngol 1993;22:331-6.
- Cohen NL. Retrosigmoid approach for acoustic tumor removal. Otolaryngol Clin North Am 1992;25:295-310.
- Eby JBC, S. T.; Shahinian, H. K. Fully Endoscopic Vascular Decompression of the Facial Nerve for Hemifacial Spasm. Skull Base: An Interdisciplinary Approach 2001;11:189-196.
- Jarrahy R, Eby JB, Shahinian HK. A new powered endoscope holding arm for endoscopic surgery of the cranial base. Minim Invasive Neurosurg 2002;45:189-92.